This application claims priority under 35 U.S.C. §119 to German Patent Application No. 10 2008 037 174.2-55, filed Aug. 11, 2008, the entire disclosure of which is herein expressly incorporated by reference.
The invention relates to a method and apparatus for enhancing the accuracy of position determination, or for reducing the integrity risk, of a receiver in a global satellite navigation system having a plurality of satellites.
In such a global satellite navigation system, the satellites communicate their exact position and the time of day to a receiver via radio. Position determination requires that the receiver simultaneously receive such signals from at least four independent satellites. The instantaneous position and time of day may be derived by computer in the receiver, by measuring the four times of receipt of the satellite signals as accurately as possible.
Since the respective location of a satellite, and thus its distance from the earth, constantly changes, these parameters cannot be determined directly by the receiver. Instead, each satellite recognizes its instantaneous position, which may be regularly compared by a base station. The distance of a satellite from the receiver is deduced from the signal transit time. Each of the satellites continuously emits its navigation data and a time signal. By comparing with its own clock, the receiver knows how long it took for the signal to reach it. The exact time may be derived from the time signals from the satellites. Therefore, position determination requires not just three, but four satellites in order to determine the three spatial coordinates (x, y, z) in the coordinate system used, as well as the exact time. The geographical longitude, geographical latitude, and the height above the defined reference ellipsoid may then be computed from the spatial coordinates. In other words, on the basis of the navigation data the receiver computes the position of the satellite within the determined or specified error limits, using an orbit model for any point in time within the period of validity of the navigation data.
One of the most important system parameters of a global navigation satellite system (GNSS) is its global availability, which is a function of various factors. It is noteworthy, however, that the signal transit time of the signals emitted by the satellites is altered by the atmosphere. This influence may be partially corrected by the fact that the receiver evaluates signals which are transmitted by a satellite at different frequencies. In principle, the accuracy of the position determination increases when the receiver is able to receive signals from more than four satellites (referred to as “overdetermined localization”); however, this is not always possible. In practice, on account of these influences the global availability cannot always be maintained at 100%.
One object of the present invention, therefore, is to provided a method and apparatus which achieve a high-integrity position solution, with a specified probability, for cases in which the global satellite navigation system is not available.
The present invention provides a method for optimizing the accuracy of the position determination, and/or for reducing the integrity risk, of a receiver in a global satellite navigation system having a plurality of satellites. In the method according to the invention, for at least one satellite that is visible to the receiver, a deviation error is determined as a function of the geometric orientation of the satellite relative to the receiver and of at least one system parameter. The deviation error is determined on the basis of an additional deviation error by means of an error projection into a coordinate system of the receiver. A first or a second value, whichever is smaller, is used as the deviation error. The first value for the at least one system parameter is determined using a respective specified parameter value. The second value is determined for the at least one system parameter, using a modified parameter value (that is, a parameter value which is modified relative to the specified parameter value in such a way that a lower error in the modified parameter value of the at least one system parameter is accepted as true).
The deviation error represents a projected error. The additional deviation error, from which the deviation error is determined, represents a so-called range error, also referred to as user equivalent range error (UERE).
The method according to the invention has the advantage that the accuracy of the position determination may be optimized and/or the integrity risk of the receiver may be reduced in a global satellite navigation system, without additional effort with regard to the necessary system components. The gain in performance may be realized solely by computational means. In this regard the invention allows a portion of the points in time, which typically would not be available, to be found as available without jeopardizing the conservative approach of the positioning. The only “costs” of the invention lie in additional computing expenditure for determining the second value of the deviation error. This determination or computation is preferably carried out in the receiver.
The invention is based on the consideration that by their nature, the navigation data transmitted via the satellites are considered to be upper or lower limits for error modeling, in order to ensure a conservative approach at any point in time. The invention is based on the fact that the position solution projected into the coordinate system of the receiver may in many cases be improved by a more extensive (i.e., increasingly conservative) degradation of the error budget (i.e., the upper or lower limits) of the at least one system parameter.
A SISA (“signal-in-space accuracy”) value, for example, may be considered as a system parameter. For example, by increasing the SISA value used in the receiver, which on the system side may be much better (i.e., smaller) in a navigation message transmitted by a particular satellite, the integrity risk of a so-called Safety of Life (SoL) service position solution may be greatly reduced. Alternatively, a SISMA value may also be considered as a system parameter. In addition, an integrity parameter may be used as a system parameter in the proposed method. Such integrity parameters are also known as “thresholds.”
It is also practical for the at least one system parameter to be transmitted to the receiver by each particular satellite.
The geometric orientation of the satellite relative to the receiver includes in particular an elevation angle and/or an azimuth angle.
The method according to the invention may be carried in a particularly simple manner and without additional system components when the deviation error (i.e., the projected error), is determined by the receiver.
According to a further advantageous embodiment, a respective deviation error is determined for all or a portion of the satellites that are visible to the receiver.
The invention further provides a device for optimizing the accuracy of position determination, and/or for reducing the integrity risk, of a receiver in a global satellite navigation system having a plurality of satellites, and means for carrying out the above-described method. In particular, this device is provided in the receiver of a global satellite navigation system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.